The invention relates to a waveguide for the transmission of electromagnetic energy, which has a low attenuation even with a small line cross-section.
The known forms of line for the transmission of electromagnetic energy can be divided, in principle, into open and shielded systems. The Sommerfeld line, the Harms-Goubau line and the dielectric line inter alia, belong to the first group, the coaxial line and the various hollow waveguides for example, belong to the second group. The coaxial cable and the rectangular waveguide, in particular, are of practical importance for relatively short transmission distances and the Harms-Goubau line and particularly the circular waveguide (H.sub.01 -wave) for low-loss transmission over greater distances and are used for long-distance traffic.
With the open line (wire waveguide) the more immediate vicinity of the conductor medium predominantly participates in the energy transport, while the line itself, merely affords a loose guiding. A prerequisite for this, however, is that the field strengths in the outside space decrease in accordance with a Hankel function with increasing distance from the conductor axis, that is to say disappear almost exponentially towards the outside. The extent of the field drop depends on the dimensions and material constants of the line and on the particular operating frequency. The great advantage of the open line (for example, the Harms-Goubau line) is known to lie in the low transmission attenuation. A disadvantage, on the other hand, is the relatively large diameter of the circular cross section which is necessary in comparison with the wavelength of operating frequency and through which 90% or 99% of the energy is transmitted, because allowance must be made for this, for example, in the mounting of the conductor (laying and supporting). A further particularly great disadvantage is the susceptibility of the open line to trouble with regard to hoarfrost and icing.
The behaviour of the coaxial line as regards attenuation is sufficiently well known. With a specific diameter ratio (.apprxeq.3.6), which is independent of the frequency, the attenuation is at a minimum. It increases proportionately to the root of the frequency and can therefore assume very high values with high frequencies. Coaxial cables are therefore used for longer transmission sections only in the range of relatively low frequencies, for example, with repeaters up to 60 MHz in carrier-frequency installations. With short and very short distances, on the other hand, where the attenuation is less important, this line is of service far into the range of microwaves. In this case, however, there is the condition that the particular operating wavelength seen electrically, must always be greater than or at least equal to the periphery of the bore of the outer conductor, because otherwise higher modes appear between inner and outer conductors and may cause disturbing effects. Variations of the coaxial line are the various conductor forms in the strip-line technique, wherein even relatively high attenuation constants can be accepted into the bargain because of the extremely short lengths.
With the tubular waveguide, the attenuation is naturally considerably less than in the coaxial line because of the large tube surface and the absense of an inner conductor. In order that the tube may be permeable to electromagnetic waves, however, its width must always be larger by a certain factor in comparison with the particular operating wavelength. With low frequencies, this leads to voluminous and expensive tube cross-sections, as for example in the type WR 650, frequency range 1.14-1.73 GHz: internal dimensions 165.1/82.55 mm, wall thickness 2.03 mm. On the other hand, for a distinct mode excitation, the operating wave length should not drop below a certain value in comparison with the critical wavelength of the tube. For very high frequencies (mm waves) this means very small tube dimensions, as a result of which there is very high attenuation, for example in the type WR10, frequency range 73.8-112.0 GHz; internal dimensions 2.54/1.27 mm, attenuation 2740 db/km at 88.6 GHz.
With the exception of the H.sub.om wave in the round waveguide, the attenuation passes through a minimum depending on the frequency in all tubular waveguides and with all modes and then increases in proportion to the root of the frequency, as in the coaxial line. The attenuation minimum is generally above the transmission range and therefore cannot be utilized. An optimum use of the tubular waveguide is, for example, where high powers also have to be transmitted at the frequency in question so that the flashover security of the wall spacing can be utilized at the same time.
In the circular waveguide, which is operated in the H.sub.om mode (circular E field) preferably in the H.sub.01 mode, it is known that the transmission attenuation decreases steadily with rising frequency. In order to obtain sufficiently low attenuation, suitable for long-distance traffic, the internal diameter of the tube must be larger by a multiple in comparison with the operating wavelength. Typical values are, for example, tube width 50-70 mm, operating frequency 60-100 GHz, transmission attenuation about 1 db/km. As a result of the relatively large diameter, numerous subsidiary modes may appear in this tube, apart from the dominant mode and may cause considerable additional losses. Their excitation is possible with the slightest deviation of the tube contour from the circular and/or straight ideal shape. Accordingly, only stable and very precisely manufactured metal tubes can be considered. Measures are also taken to decouple certain modes. In particular, these are a thin dielectric wall coating or the covering of the inner wall of the tube with a tightly wound coil of thin, enamelled copper wire. With the dielectrically coated tube, H.sub.01 wave purification is also necessary by means of mode filter disposed at intervals, the proportion of which may amount to 2-25% of the total line length, depending on tube tolerances. In addition, a very stable laying of the line is necessary, for example, resilient embedding in protective tubes (tube-in-tube laying). Thus the use of the circular waveguide (hollow cable) for long-distance traffic is very expensive.
In general, with all conventional forms of line, a relatively large field cross-section is always necessary for a low-loss transmission. The practical use of such lines is therefore associated with great disadvantages, as the above explanations show, particularly for long-distance traffic, with regard to handling, technical and cost expense. This is obviously an important reason why today the line transmission, for example, of microwaves, has not become very widespread.
The transmission of intelligence by means of glass optical fibers is at present being fully developed. Attenuation of 5-10 db/km are expected. The long-term behavious of the fibers is unknown. Even slight opacity would have a disastrous effect on the attenuation. Also the available light efficiencies are still comparatively low, particularly in the single-fiber technique, so that the signal-to-noise ratios are lower by about 30 db than can be achieved by conventional means in communication channels.